VDPV AND VAPP

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OPV (ORAL POILIO VACCINE)



Oral polio vaccine (OPV) contains an attenuated (weakened) vaccine-virus, activating an immune response in the body. 

When a child is immunized with OPV, the weakened vaccine-virus replicates in the intestine for a limited period, thereby developing immunity by building up antibodies. 

During this time, the vaccine-virus is also excreted. In areas of inadequate sanitation, this excreted vaccine-virus can spread in the immediate community (and this can offer protection to other children through ‘passive’ immunization), before eventually dying out.

VACCINE DERIVED PARALYTIC POLIOMYELITIS(VDPV)

On rare occasions, if a population is seriously under-immunized, an excreted vaccine-virus can continue to circulate for an extended period of time. 

The longer it is allowed to survive, the more genetic changes it undergoes. 

In very rare instances, the vaccine-virus can genetically change into a form that can paralyse – this is what is known as a circulating vaccine-derived poliovirus (cVDPV).

It takes a long time for a cVDPV to occur. Generally, the strain will have been allowed to circulate in an un- or under-immunized population for a period of at least 12 months. 

Circulating VDPVs occur when routine or supplementary immunization activities (SIAs) are poorly conducted and a population is left susceptible to poliovirus, whether from vaccine-derived or wild poliovirus. 

Hence, the problem is not with the vaccine itself, but low vaccination coverage. 

If a population is fully immunized, they will be protected against both vaccine-derived and wild polioviruses.

Currently, the type 2 component contained in trivalent OPV accounts for more than 90% of all cVDPV cases (bivalent OPV does not contain type 2).

3 types of VDPP

Circulating cVDPV
Immunodeficiency VDPV
Ambiguous

Circulating immunodeficiency is the one in which the evidence of virus circulation is present.

Immunodeficiency-related vaccine-derived poliovirus (iVDPV) is a type of VDPV in which individuals with a primary immunodeficiency disorder (PID) excrete Sabin polioviruses; in some cases, for substantially longer periods than immunocompetent individuals.

 After exposure to OPV, immunocompetent individuals usually excrete the vaccine virus for 4–8 weeks .

However, in immunodeficient individuals, an inability to mount an adequate immune response can result in persistence of the intestinal infection with poliovirus and prolonged viral shedding.

In the process, the virus can mutate to re-acquire the neurovirulence and transmissibility characteristics of wild poliovirus  

VACCINE ASSOCIATED PARALYTIC POLIOMYELITIS

OPV is made with live attenuated (weakened) polioviruses that can result in a case of vaccine-associated paralytic polio (VAPP) in approximately 1 in 2.7 million doses of OPV. 

VAPP is caused by a strain of poliovirus that has genetically changed in the intestine from the original attenuated vaccine strain contained in OPV. 

It can occur among otherwise healthy OPV recipients, or healthy person who had close contact with vaccine recipients, or community contacts as well. 

Although there is no standard case definition for VAPP, a case of recipient VAPP is generally accepted if acute flaccid paralysis occurred 4 to 40 days after receiving OPV and residual weakness lasted for more than 60 days after the onset of paralysis. 

Isolation of vaccine-related poliovirus in any stool samples and no isolation of wild poliovirus from any stool samples are also generally required to confirm the VAPP cases.

APP cases are also be categorized as either: 1) a recipient case: the patient must have received OPV 4 to 40 days prior to the onset of illness, or 2) a contact case: the patient must have had a contact with recent OPV recipients who received immunization 4 to 75 days before the onset of illness

 VDPV         VAPP
 It occurs in a wide population    as epidemic it is a single event in an individual receiving vaccine or a very close contact

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